Zeolite catalysts have become widely used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation, transalkylation, isomerization, polymerization, addition, disproportionation and other acid catalyzed reactions may be performed with the aid of these catalysts. Certain natural and synthetic zeolites are known to be active for reactions of these kinds.
The common crystalline zeolite catalysts are the aluminosilicates such as Zeolites A, X, Y and mordenite. Structurally each such material can be described as a robust three dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra that is crosslinked by the sharing of oxygen atoms whereby the ratio of total aluminum and silicon atoms to oxygen is 1:2. These structures (as well as others of catalytic usefulness) are porous, and permit access of reactant molecules to the interior of the crystal through windows formed of eight-membered rings (small pore) or of twelve-membered rings (large pore). The electrovalence of the aluminum that is tetrahedrally contained in the robust framework is balanced by the inclusion of cations in the channels (pores) of the crystal.
An "oxide" empirical formula that has been used to describe the above class of crystalline zeolites is EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M is a cation with valence n, x has a value of from 2 to 10, and y has a value which varies with the pore volume of the particular crystal under discussion. The above oxide formula may be rewritten as a general "structural" formula EQU M.sub.2/n (AlO.sub.2.wSiO.sub.2)yH.sub.2 O
wherein M and y are defined as above, and wherein w has a value from 1 to 5. In this representation, the composition of the robust framework is contained within the parenthesis, and the material (cations and water) contained in the channels is outside the parenthesis. One skilled in the art will recognize that x in the empirical oxide formula represents the mole ratio of silica to alumina in the robust framework of a crystalline zeolite, and shall be referred to herein simply by the expression in common usage, i.e. "the silica to alumina ratio". (See "Zeolite Molecular Sieves", Donald W. Breck, Chapter One, John Wiley and Sons, New York, N.Y. 1974, which is incorporated herein by reference as background material).
With few exceptions, such as with Zeolite A wherein x=2, there are fewer alumina tetrahedra than silica tetrahedra in the robust framework. Thus, aluminum represents the minor tetrahedrally coordinated constituent of the robust framework.
It is generally recognized that the composition of the robust framework may be varied within relatively narrow limits by changing the proportion of reactants, e.g., increasing the concentration of the silica relative to the alumina in the zeolite synthesis mixture. However, definite limits in the maximum obtainable silica to alumina ratio are observed. For example, synthetic faujasites having a silica to alumina ratio of about 5.2 to 5.6 can be obtained by changing said relative proportions. However, if the silica proportion is increased above the level which produces the 5.6 ratio, no commensurate increase in the silica to alumina ratio of the crystallized synthetic faujasite is observed. Thus, the silica to alumina ratio of about 5.6 must be considered an upper limit in a preparative process using conventional reagents. Corresponding upper limits in the silica to alumina ratio of mordenite and erionite via the synthetic pathway are also observed. It is sometimes desirable to obtain a particular zeolite, for any of several reasons, with a higher silica to alumina ratio than is available by direct synthesis. U.S. Pat. No. 4,273,753 to Chang and the references contained therein describe several methods for removing some of the aluminum from the framework, thereby increasing the silica to alumina ratio of a crystal.
For the above zeolite compositions, wherein x has a value of 2 to 10, it is known that the ion exchange capacity measured in conventional fashion is directly proportional to the amount of the minor constituent in the robust framework, provided that the exchanging cations are not so large as to be excluded by the pores. If the zeolite is exchanged with ammonium ions and calcined to convert it to the hydrogen form, it aquires a large catalytic activity measured by the alpha activity test, which test is more fully described below. And, the ammonium form of the zeolite desorbs ammonia at elevated temperature in a characteristic fashion.
Synthetic zeolites wherein x is greater than 12, which have little or substantially no aluminum content, are known. Such zeolites have many important properties and characteristics and a high degree of structural stability such that they have become candidates for use in various processes including catalytic processes. Materials of this type are known in the art and include high silica content aluminosilicates, such as ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No. 3,709,979), and ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few. Unlike the zeolites described above wherein x=2 to 5, the silica to alumina ratio for at least some of the high silica content zeolites is unbounded. ZSM-5 is one such example wherein the silica to alumina ratio is at least 12. U.S. Pat. No. 3,941,871 discloses a crystalline metal organosilicate essentially free of aluminum and exhibiting an X-ray of diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe microporous crystalline silicas or organosilicates wherein the alumina content present is at very low levels. Some of the high silica content zeolites contain boron or iron which is not reversibly removed by simple ion exchange, i.e. the zeolites contain tenaciously bound boron or iron.
Because of the extremely low alumina content of certain high silica content zeolites, their catalytic activity is not as great as materials with a higher alumina content. Therefore, when these materials are contacted with an acidic solution and thereafter are processed in a conventional manner, they are not as catalytically active for acid-catalyzed reaction as their higher alumina content counterparts.
U.S. Pat. No. 4,380,685 teaches para-selective alkylation, transalkylation or disproportionation of a substituted aromatic compound to form a dialkylbenzene compound mixture over catalyst comprising zeolite characterized by a constraint index 1 to 12 and a silica:alumina mole ratio of at least 12:1, the catalyst having thereon incorporated various metals and phosphorus. Other patents covering alkylation and transalkylation include U.S. Pat. Nos. 4,127,616, 4,361,713, 4,365,104, 4,367,359, 4,370,508 and 4,384,155. Toluene is converted to para-xylene in U.S. Pat. Nos. 3,965,207, 3,965,208, 3,965,209, 4,001,346, 4,002,698, 4,067,920, 4,100,215 and 4,152,364, to name a few. Alkylation with olefins is taught, for example, in U.S. Pat. Nos. 3,962,364 and 4,016,218 and toluene is disproportionated in, for example, U.S. Pat. Nos. 4,052,476, 4,007,231, 4,011,276, 4,016,219 and 4,029,716. Isomerization of xylenes is taught in, for example, U.S. Pat. Nos. 4,100,214, 4,101,595, 4,158,676, 4,159,282, 4,351,979, 4,101,597, 4,159,283, 4,152,363, 4,163,028, 4,188,282 and 4,224,141.